Not Applicable.
Not Applicable.
Dissolved metal reduction reactions have been used in the field of organic chemistry for 150 years. This type of reaction involves placing a compound in contact with a reductive metal in order to chemically reduce the compound. Typical dissolved metal reduction reactions involve the use of lithium, sodium or aluminum which have strong electrode potentials of 3.03, 2.71 and 1.66 volts, respectively. It would be desirable to utilize these metals for reductive treatment of organic and inorganic pollutants for remediation of groundwater and soils. However, these metals reduce water into hydrogen gas and hydroxide anions which interfere with chemical reduction of pollutants by fouling the surface of the metal with a film of hydrogen gas and hydroxide precipitates. Therefore, much attention has been focused on identifying alternate metals that are capable of achieving beneficial reductive reactions with minimal formation of hydrogen gas and hydroxide precipitates at the metal surface.
The periodic table contains 78 metals that may be evaluated for use in dissolved metal reduction reactions of pollutants. The six alkali metals and six alkaline earth metals all have electrode potentials that exceed 0.83 volts. Therefore, these metals are unsuitable for long-term reductive performance because they reduce water into hydrogen gas and hydroxide which results in fouling at the surface of the metal. Twelve additional transition metals also have electrode potentials that exceed 0.83 volts, making them also unsuitable for use in remedial applications. Other transition metals are cathodic and have an electrode potential less than negative 0.5 volts. These metals include palladium, mercury, osmium, silver, gold, iridium, platinum and technetium. These metals are highly resistant to oxidation and would not be able to spontaneously reduce pollutants in groundwater or soils. Finally, the list of 28 inner-transition metals cannot be used due to high reactivity or radioactive decay. These data show that more than 75 percent of the metals in the periodic table are unsuitable or incapable of being used for dissolved metal reductive reactions for remedial purposes. Of the few remaining suitable metals that may be used most of the remedial experimentation has focused on the use of iron or zinc due to the low toxicity and high availability of these metals.
Initial work in the remedial field of dissolved metal reduction reactions was pioneered by K. H. Sweeny in 1972. Sweeny used zinc metal to reductively dechlorinate pesticides in the laboratory in a process later patented in 1972 (U.S. Pat. No. 3,640,821). In 1980, Sweeny expanded his use of remedial dissolved metal reduction to include the use of iron for reduction of chlorinated solvents in industrial waste water (Sweeny, K. H., The Reductive Treatment of Industrial Wastewaters, AMERICAN INSTITUTE OF CHEMICAL ENGINEERS, SYMPOSIUM SERIES 209, WATER-1980, Ed. G. F. Bennett, Vol. 77, pp 67-78).
In 1992, Gillham and O""Hannesin began to further investigate the use of iron and zinc fines for in-situ remedial treatment applications after they observed such reductive treatment occurring in monitoring wells constructed of galvanized steel. Gillham and O""Hannesin extended their studies to include bodies of metal such as brass and copper but found that these metals were substantially less effective than iron and zinc (Gillham, R. W. and O""Hannesin, S. F., Enhanced Degradation of Halogenated Aliphatics by Zero-Valent Iron, GROUNDWATER, Vol. 32, pp 958-967). Gillham and O""Hannesin also reported that no degradation of chlorinated solvents was observed when the body of metal used for treatment was stainless steel This presents a fictional issue because metals such as zinc and iron perform well for reductive dechlorination but are subject to oxide fouling when exposed to water and oxygen. On the other hand, metal alloys such as stainless steel are resistant to oxide fouling, but they are non-functional for use in dissolved metal reduction reactions. In 1992, Robert Gillham filed a patent that involved the use of metal for subsurface remediation where the body of metal is handled in a manner that prevents substantially all traces of oxygen from reaching an anaerobic portion of the body of metal (U.S. Pat. No. 5,266,213). It would be desirable to generate a metallic surface with an electrode potential significant enough to reduce pollutants that is also resistant to oxygen corrosion under neutral or slightly acidic groundwater conditions.
This invention describes a method for reductive treatment of pollutants in groundwater, soil, waste or water using an iron-impregnated, carbon-coated silica sand. The iron contains between 1.7 and 4.5 percent carbon and between 2 and 16 percent silicon to prevent aerobic corrosion at the surface of the metal. The silica sand also contains up to four percent carbon by weight to facilitate adsorptive fixation and retardation of pollutants at the surface of the iron-impregnated sand.
Accordingly, several objects and advantages of our invention are:
(a) the invention provides a method of performing reductive remediation with dissolved metal reactions while minimizing interferences caused by oxide fouling due to dissolved oxygen in water or exposure to oxygen in the atmosphere;
(b) the invention provides a media with high surface area for adsorptive fixation and reductive treatment;
(c) the invention provides higher than anticipated reaction kinetics while utilizing only low concentrations of iron in the media; and
(d) the invention provides an inexpensive media for treatment of pollutants since the matrix is primarily composed of sand.
Further objects and advantages of our invention will become apparent from a consideration of the drawings and ensuing description.